The invention is in the field of semiconductor technology and relates to a method for fabricating microstructures.
Advancing scaling of integrated circuits and the associated miniaturization and increasing complexity of the structures contained in the integrated circuits entail a multiplicity of problems.
Thus, by way of example, the interconnect resistances of the individual interconnects within the wiring planes increase. The capacitive coupling between the interconnects increases at the same time. Associated with this is an unfavorable rise in the RC time constants, which lead to rising signal propagation times, decreasing limiting frequencies, high power loss densities and crosstalk effects. The technical literature also groups these problems under the term xe2x80x9cwiring crisisxe2x80x9d. In this respect, see, in particular, the technical articles by M. T. Bohr, xe2x80x9cInterconnect Scalingxe2x80x94The Real Limiter to High Performance ULSIxe2x80x9d, IEDM 95, pages 241-244; S. Oh and K. Chang, xe2x80x9c2001 Needs for Multi-Level Interconnect Technologyxe2x80x9d, Circuits and Devices, January 1995, pages 16-20; T. H. Ning, xe2x80x9c0.1 xcexcm Technology and BEOLxe2x80x9d, Mat. Res. Soc. Symp. Proc., Vol. 427, 1996, pages 17-21; and K. Yamashita and S. Odanaka, xe2x80x9cInterconnect Scaling Scenario using a Chip Level Interconnect Modelxe2x80x9d, Symp. On VLSI Technology Digest of Technical Papers, 1997, pages 53-54.
A further problem associated with miniaturization can be examined in the proximity effects of lithography (e.g. U.S. Pat. No. 5,821,014, whose disclosure is incorporated herein by reference thereto) and etching technology. In the fabrication of integrated circuits, these effects lead to structural results which deviate from the desired structures prescribed on the lithography mask. In order to reduce or eliminate the proximity effects, it is necessary to resort to corrections (for example layout changes, U.S. Pat. No. 6,083,275, whose disclosure is incorporated herein by reference thereto) of ever increasing complexity. The effects discussed are based on the fact that individual process steps of the overall patterning process (exposure, development, plasma etchings, chemical mechanical polishing, etc.) depend on the local structural characteristic features (packing density, distances from adjacent structures, center and edge positions). The required layout changes have to be determined for each new layout, with a high outlay, and taken into account by means of suitable measures on the corresponding masks.
In order to reduce the RC time constants, use is often made of interconnects having high conductivity (e.g. copper) and dielectrics having low dielectric constants ∈r (often also designated by k). Diverse organic dielectrics having dielectric constants of approximately 2:5 are used as xe2x80x9clow-k dielectricsxe2x80x9d (e.g. so-called aerogels or xerogels, U.S. Pat. No. 6,277,728, whose disclosure is incorporated herein by reference thereto), but they are usually associated with diverse disadvantages (low thermal and chemical stability, hygroscopy, low blocking effect against the diffusion of e.g. copper, need to develop new deposition and patterning processes, new process installations). Porous inorganic dielectrics having even lower dielectric constants (e.g. porous SiON) are not yet ready for production at the present time.
The best material with extremely low ∈r is a vacuum or air (∈r=1). Therefore, a series of documents deal with techniques enabling the production of cavities between interconnects.
Thus, in accordance with U.S. Pat. No. 6,252,290 B1, whose disclosure is incorporated herein by reference thereto, by way of example, after the completion of the interconnects, cavities are produced between the interconnects by subsequent removal of insulation material.
U.S. Pat. No. 5,936,295, whose disclosure is incorporated herein by reference thereto, by contrast, exploits the porosity of a dielectric layer. After the completion of interconnect sections between which an auxiliary material is situated, the dielectric layer is applied to the interconnect sections and the auxiliary material. On account of the porosity of the dielectric layer, an etching substance can penetrate as far as the auxiliary material and remove the latter. As a result, cavities remain between the interconnect sections.
A similar approach is pursued by U.S. Pat. No. 6,297,125 B1, whose disclosure is incorporated herein by reference thereto, but here a dielectric covering layer with subsequently introduced slots is used, through which the etching substance can penetrate as far as the auxiliary material.
In contrast thereto, in accordance with U.S. Pat. No. 5,869,880, whose disclosure is incorporated herein by reference thereto, in each patterning plane (metallization and contact hole plane), the initially solid dielectric between the metal structures is finally patterned, thereby forming comparatively small openings in the dielectric in comparison with the metal structures, which openings contribute to a reduction of the dielectric constants taking effect.
An additional method for forming cavities between metal structures is disclosed in United States Patent Application Publication No. US 2001/0002732A1, whose disclosure is incorporated herein by reference thereto and which claims priority from DE 199 57 302 A1, wherein the cavities can be produced in a self-aligned manner with respect to the metal structures.
The above-mentioned methods have the disadvantage of a relatively high process complexity. Despite the cavities, capacitive couplings occur between the interconnects of different metallization planes on account of the dielectric layers (e.g. silicon oxide or silicon nitride) situated there. One possibility of reducing these capacitive couplings is afforded by the production of macroscopic cavity arrangements in these dielectric layers, i.e. in the contact hole plane, as is described for example in the German patent application DE 101 09 778.6 from the same applicant and in previously-cited U.S. Pat. No. 5,869,880.
However, the fundamental problems going back to the proximity effects of lithography and etching technology are not avoided with the abovementioned methods.
Therefore, it is an object of the present invention to specify a method in which the effects of proximity effects are considerably reduced.
This object is achieved by means of a method for fabricating microstructures comprises the following steps:
a) forming of a regular arrangement of closed cavities in a patterning layer arranged on a substrate, the cavities being separated from one another by partitions;
b) opening of selected cavities in which microstructures are intended to be produced;
c) removal of at least a portion of the partitions situated between adjacent opened cavities in order to form trenches extending over at least two cavities; and
d) introduction of a material into the trenches in order to fabricate the microstructures.
Accordingly, in accordance with the invention, firstly a regular arrangement of closed cavities is formed in a patterning layer. Regular structures can be fabricated with particularly high precision and accuracy in conjunction with a comparatively small extent. On account of the regularity, no local differences occur during the exposure and the etching. Therefore, local proximity effects are avoided particularly during a global regular patterning of the patterning layer. On the other hand, the regularity of the cavities also allows the imaging of cavities which are so small that they can no longer be imaged satisfactorily in the case of separate imaging. The individual cavities are separated from one another by partitions comprising the material of the patterning layer.
In order to reliably ensure the regularity, the closed cavities are formed by microelectronic processes known per se (e.g. CVD, lithography, etching, CMP). It is preferably the case, therefore, that in order to form the cavities, depressions are produced in the patterning layer and are closed with a covering layer, thereby producing the cavities. In this case, the dimensions of the depressions or the cavities are of the order of magnitude of the microstructures to be produced. In particular, the minimum lateral extent of the microstructures to be produced is determined by the extent of the cavities.
Preferably, the cavities or the depressions etched into the patterning layer in order to form the cavities are defined lithographically and subsequently etched into the pattering layer. In this case, a resist mask may preferably be used for the lithographic definition of the cavities or the depressions, at least two regular strip patterns rotated through a predetermined angle relative to one another being successively imaged into said resist mask. In contrast to raster-shaped patterns, strip patterns can even be imaged with higher accuracy in conjunction with the same resolution. On the other hand, they also enable the imaging of more closely adjacent lines than is possible in the case of a raster-shaped pattern. The possibilities of lithography can be maximally utilized by means of the imaging of strip patterns.
During the imaging of regular strip patterns, at least two exposures are required, the strip pattern being rotated through a predetermined angle, e.g. 90xc2x0, between the individual exposures. In principle, it is also possible to image different (e.g. with a different line spacing) strip patterns rotated with respect to one another, in order, for example, to produce cavities that are rectangular in cross section.
It is favorable if,
in order to form the patterning layer, at least a first and a second layer are successively applied to the substrate, the materials of the first and second layers being chosen so as to enable a deposition of the material of the covering layer on the second layer selectively with respect to the first layer,
in order to form the cavities, the depressions are formed in the first and second layers, the depressions penetrating through the second layer completely and penetrating through the first layer at least partially, and, afterward, the material of the covering layer is deposited in such a way that it grows essentially only on the second layer,
since this enables the covering layer to be applied in a particularly simple and purposeful manner. Moreover, inadvertent filling of the depressions is reliably avoided. Accordingly, the materials for the first and second layers are selected so as to ensure a selective deposition of the covering layer on the second layer. The depressions are formed completely in the second layer and at least partly in the first layer. Preferably, the first layer is likewise etched through completely. The residual material between the depressions forms the partitions.
A third layer may be applied in addition to the first and second layers, the material of which third layer differs from the second layer, so that the material of the covering layer essentially likewise grows only on the second layer. Accordingly, no material of the covering layer grows on the first and third layers, even if, during the progressive growth of the material of the covering layer on the second layer, the third layer is possibly covered by said material. In the case of a three-layered construction of the patterning layer, the third layer preferably serves as a polishing stop during the subsequent polishing-back of the protruding material of the covering layer. In the case of a two-layered construction, by contrast, the second layer preferably performs this function.
The first, second and third layers are preferably dielectric layers, since, for dielectric layers, it is possible to have recourse to a multiplicity of suitable and proven patterning methods, in particular anisotropic plasma etching methods.
The use of silicon nitride for the first and third layers is particularly favorable. By contrast, a silane-based silicon oxide deposited in plasma-enhanced fashion is preferably used for the second layer. Given this choice of materials for the first, second and third layers, an oxide can be deposited selectively on the second layer by means of O3/TEOS deposition. Therefore, an oxide of this type is preferably used for the covering layer.
In order to reliably ensure the formation of the cavities, the first layer is preferably made about 4 to 5 times thicker than the second layer.
Afterward, those cavities in which the microstructures to be fabricated are intended to be formed are opened at their top side preferably by methods of lithography and etching technology. To that end, a mask (photomask/resist mask) is applied to the covering layer, leaving uncovered the cavities that are to be opened. The material of the covering layer is expediently etched selectively with respect to the material of the first, second and third layers, and the material of the mask. As a result, even in the event of a possible misalignment of this mask with regard to the cavities, the side walls of the cavities are not etched, so that the position and extent of the cavities are preserved. The mask (for example the strip masks) used for forming the cavities (or the depressions) is, accordingly, the actual patterning mask, whereas the mask for opening the cavities performs the function of a selection mask. Therefore, less stringent requirements can be made of the mask for opening the cavities than of the mask for defining the cavities.
At least a portion of the microstructures is intended to extend over a plurality of opened cavities. This requires the removal of the webs or partitions of the patterning layer which are situated between the opened cavities. In this case, recourse is preferably had to the same mask (selection mask) which has already been used for opening the closed cavities. Through the removal of the webs or the partitions, adjacent cavities that are initially separated from one another are combined to form individual trenches running in the patterning layer. The fabrication of the microstructures is concluded by filling the opened cavities or the trenches with the material of the microstructures to be fabricated and, if appropriate, polishing-back (for example by means of CMP) the filling material.
It lies within the scope of the invention to fabricate by the method according to the invention exclusively those microstructures which extend over at least two cavities, and, in addition thereto, to fabricate those microstructures which only fill individual opened cavities. In the latter case, in addition to the trenches, individual opened cavities are also filled with the material of microstructures in order to form said microstructures.
The cavities have a lateral extent prescribed in particular by the lithography used (masks). In this case, in the context of the invention, lateral extent is understood to mean that which extends in the plane of the patterning layer. In the simplest case, the cavities have a square or approximately square cross section in a plan view of the patterning layer, deviations from the square cross section being caused in particular as a result of the imperfection during the lithographic imaging. If the microstructures to be fabricated are formed in a trench encompassing two adjacent cavities, their lateral extent at least in one direction amounts to at least twice the lateral extent of the cavities in this direction.
The method according to the invention may also be referred to as a xe2x80x9ccellular damascene methodxe2x80x9d. The basic concept is fabricating the microstructures by selecting individual cavities and, if appropriate, combining a plurality of cavities. Each cavity forms one cell in this case. In this case, the arrangement and the course of the microstructures to be fabricated depend on the raster-shaped arrangement of the cells. The latter may be made square, for example, and be arranged in rows and lines. Other arrangements are likewise possible.
The microstructures to be fabricated can ultimately be any desired microstructures, i.e. both microelectronic and micromechanical structures. The type of microstructures to be fabricated also decides the material with which the uncovered cavities or the trenches are filled. The material may be e.g. metals, insulators, semiconductors, ferroelectrics etc. The fabrication of interconnects is particularly preferred, so that the material to be introduced is, in particular, metals and/or metal alloys, and preferably copper.
The method according to the invention also at the same time alleviates the problem of capacitive coupling in particular between interconnects, since, through selection of the cavities to be opened, the interconnects can be placed so that closed cavities remain between adjacent interconnects. Since the dielectric constant ∈r of said cavities amounts to 1, the dielectric constants taking effect between the adjacent interconnects is correspondingly reduced.
The advantages that can be achieved with the method according to the invention can be summarized in particular as follows:
The regular arrangement of the cavities gives rise to microstructures which do not depend on proximity effects. The line widths and lengths are equal to the cell width or a multiple thereof if appropriate plus the web width.
Steepness and accuracy of the microstructures are determined, independently of the material to be introduced, essentially by the cavity side walls, that is to say primarily by patterning methods used to form the depressions or cavities, in particular tried and tested anisotropic plasma etchings of dielectrics (silicon oxide, silicon nitride). The side walls can therefore be steep and smooth for all materials of the microstructures to be fabricated.
The distance between the microstructures is determined by the web width of the cells alone or by the web width and the cavity width.
Complex proximity corrections are obviated owing to the regular arrangement of the cells. Length and width of the resulting microstructures are an integer multiple of the cell size plus the width of the partitions originally situated between the combined cells. If the cells are rectangular, the length and the width of the microstructures in each case amount to the integer multiple of the lateral extent of the cells in the corresponding direction.
Cavities between the interconnects make it possible to reduce the capacitive couplings between the interconnects and thus generally capacitive couplings and signal delays.
It is possible to have recourse to all the advantages of the damascene method, e.g. patterning of materials that cannot be etched or are difficult to etch.
The size of the microstructures is preferably in the micrometers range, i.e. 1-1000 xcexcm, or less ( less than 1 xcexcm). The method is preferably used in the fabrication of integrated circuits.
The invention furthermore relates to an arrangement of microstructures having a patterning layer in which a multiplicity of closed cavities and a multiplicity of microstructures are arranged, the closed cavities being seated on grid points of a predetermined raster and the microstructures running along the connecting lines that connect the grid points, and the closed cavities having a given lateral extent in at least one direction and the lateral extent of the microstructures in this direction corresponding to at least twice the lateral extent of the cavities.
In the arrangement according to the invention, the orientation and the position of the individual microstructures are prescribed by the raster of the cavities. Wherever microstructures are situated, there were originally cavities present. Combining at least two adjacent cavities, i.e. removing the partitions or webs situated between the cavities, has resulted in a continuous trench filled with the material of the microstructures. The partitions running between adjacent cavities and between cavities and microstructures adjacent thereto preferably have the same thickness, the partitions running parallel to the connecting lines and in a manner offset uniformly with respect thereto.
The closed cavities preferably have a given lateral extent, so that the length of the microstructures corresponds to at least twice the extent of the cavities, if appropriate plus the width of the partition originally present between the adjacent cavities. The minimum extent of the microstructures preferably corresponds to the extent of the cavities.
It is furthermore preferred for the grid points to have a distance D along a first direction and the closed cavities to have a lateral extent B along said direction, where the length of the microstructures is nxc2x7D+B and n is natural number (n greater than 0). If n=1, by way of example, the microstructure extends over two adjacent cavities. It lies within the scope of the invention that the grid points may have a different distance D1 and D2 and a different lateral extent B1 and B2 in a different direction, e.g. perpendicularly to one another. This may be the case e.g. with cavities having a rectangular cross section in a plan view of the patterning layer. The microstructures then have an extent of nxc2x7D1+B1 and mxc2x7D2+B2 in the respective direction, where m and n are natural numbers and at least one of these two numbers is greater than 0. By way of example, it may be that m=0, and n greater than 0. In this case, the width of the microstructures corresponds to the lateral extent B2 and the length corresponds to a multiple of D1 plus B1. The microstructures can assume practically any desired form within the raster prescribed by the cavities. Strip-shaped, areas and angular forms shall be mentioned here as examples. The cavities preferably have a square cross section in a plan view of the patterning layer, so that D1=D2=D and B1=B2=B hold true.
Preferably, the patterning layer comprises at least a first and a second layer made of different material, said second layer lying on said first layer, and the cavities run at least in the first layer and the microstructures run at least in both layers. A covering layer closing the cavities is preferably arranged in the plane of the second layer, so that the cavities essentially extend below the second layer, i.e. essentially within the first layer. By contrast, the microstructures preferably penetrated through both the first and the second layer, and preferably terminate with the upper edge of the second layer. If the patterning layer is even provided with at least three layers, the microstructures extend over all three layers. The cavities can be formed completely or only partly in the first layer, i.e. they did not completely penetrate through the first layer on the side remote from the second layer.
Preferably, the material of the first layer is silicon nitride and the material of the second layer is silicon oxide. The microstructures preferably constitute interconnects, in which case they comprise, in particular, a metal or a metal alloy, and preferably copper.
The covering layer closing the cavities is preferably grown selectively on the material of the second layer.
The arrangement according to the invention preferably involves microstructures and in particular interconnects of an integrated circuit.
The invention is described below on the basis of exemplary embodiments and is illustrated in the Figures.